A manufacturing method of a spark plug is disclosed in Patent Document No. JP-A-2005-340171. The spark plug has a configuration including an insulator having a through-hole in an axial direction, a center electrode inserted into and fixed to a leading end side of the through-hole, a terminal electrode inserted into and fixed to a rear end side of the through-hole, a first conductive sealing material layer fixed to the center electrode in the through-hole, a second conductive sealing material layer fixed to the terminal electrode in the through-hole, and a resistor interposed between the first conductive sealing material layer and the second conductive sealing material layer in the through-hole and fixed to both conductive sealing material layers.
The spark plug is manufactured by the following two processes. First, in the first process, after inserting the center electrode into the leading end side of the through-hole, from the rear end side of the through-hole, predetermined amounts of a first powder material that is to be the first conductive sealing material layer, a second powder material that is to be the resistor, and a third powder material that is to be the second conductive sealing material layer are sequentially charged and compacted to cause solidification, and the terminal electrode is inserted from the rear end side of the through-hole until it abuts the third powder material and then stops.
Thereafter, in the second process, the terminal electrode is inserted to a predetermined position in a state where the insulator is heated to a temperature equal to or greater than the softening temperatures of the first to third powder materials. Accordingly, the first powder material becomes the first conductive sealing material layer, the second powder material becomes the resistor, and the third powder material becomes the second conductive sealing material layer.
In the case where the spark plug is manufactured as described above, since the first to third powder materials are compacted by the terminal electrode in the second process, a large amount of stress occurs in the insulator. Therefore, there is a concern that the insulator will suffer breakage. Recently, because a reduction in the size or diameter of the spark plug has been required, the thickness of the insulator has been reduced. As a result, the danger of this happening (i.e., breakage) is particularly high.
As a unit for avoiding insulator breakage, uniformly reducing the insertion speeds of the terminal electrode can be considered. However, in this case, although the decrease in yield due to breakage can be avoided, the time needed for the second process increases, so that productivity is degraded.
In order to solve the above-mentioned problems, it is an object of the invention to simultaneously avoid a decrease in yield due to breakage and degradation of productivity.